The concept of using geosynthetic clay liners made with bentonite for lining landfill sites is well-known. One problem addressed by the present invention involves the prevention of bentonite falling or extruding out of the edges of the geosynthetic liners and migrating into the drainage system. Activated bentonite will clog a drainage system rendering it ineffective and therefore a bentonite liner that prevents this occurrence is desired.
Another related problem addressed by the present invention is the use of a primary carrier sheet with a high enough modulus so it will not stretch downward under force and engage the drainage system thereby reducing the cross sectional area otherwise available for drainage of leachate.
Bentonite is a clay material that expands and becomes substantially impermeable upon being exposed to water. When a layer of dry bentonite is wetted, the expanding bentonite will form a low permeability barrier. However, a problem is created when the expanding bentonite is not effectively contained and allowed to migrate downward toward the drainage system. The activated bentonite will clog a drainage system.
Many modern landfills are constructed with an underlying drainage system. Drainage systems may come in the form of a drainage net or a layer of rock, aggregate or sand. A drainage system is intended to prevent the accumulation of water or other liquid, sometimes referred to as leachate, at the bottom of the landfill. Further, a drainage system disposed beneath the landfill enables environmental regulators to test for the leakage of toxic or other hazardous materials from the landfill itself.
Therefore, in some landfill designs, it is essential to keep the drainage system unclogged to allow for the drainage of liquid beneath the bottom of the landfill. In order to alleviate the problem of bentonite clogging the drainage system, some landfill owners have installed an extra geotextile layer, or textile layer, below the bentonite liner to protect the drainage system. The extra geotextile layer is expensive in terms of both materials and labor.
Thus, there is a need for a geosynthetic liner that contains wastes and hazardous materials and prevents substantial downward migration of activated bentonite toward the drainage system. Such a bentonite liner with drainage system protection will eliminate the need for an additional layer of geotextile disposed below the bentonite resulting in considerable cost savings to the landfill owner or contractor.
A geosynthetic clay liner (GCL) normally includes: (1) a primary carrier sheet, also known as a primary backing, a primary textile or a primary carrier; (2) a cover sheet, also known as a secondary textile, a secondary carrier sheet or scrim; and (3) a layer of bentonite disposed therebetween.
In terms of industry nomenclature, the terms primary textile, primary backing, primary carrier and primary carrier sheet are often used interchangeably. Unless otherwise noted, the primary carrier sheet will refer to the more durable textile or sheet of a geosynthetic liner and is normally disposed downward upon installation. However, primary carrier sheets may be disposed upward upon installation, depending upon the intended purpose of the geosynthetic liner. Also, the terms secondary textile, secondary carrier sheet, scrim and cover sheet may also be used interchangeably. Unless otherwise note, the term cover sheet will refer to the textile or sheet that is connected or otherwise attached to the primary carrier sheet with a layer of bentonite disposed therebetween. Normally, the cover sheet is disposed upward upon installation, but may also be disposed downward, depending upon the intended purpose of the geosynthetic liner.
The layer of bentonite may be held in place by: gluing the bentonite to the primary carrier sheet or the cover sheet; or the bentonite may be suspended between the two sheets and held in place by needle-punched threads extending from one sheet to the other sheet; or the bentonite may be held in place by stitch bonding the cover sheet to the primary carrier sheet; or any combination of the above. The cover sheet may be made from material that is of a lighter weight than the primary carrier sheet if its main purpose is to protect the bentonite layer during installation. Further, geosynthetic clay liners within the scope of this invention may include a primary carrier sheet only, and no cover sheet. Depending upon the design of the landfill, the cover sheet as well as the primary carrier sheet may be disposed up or down after installation.